The present invention relates to a method of improving certain properties of nanotube and nanoparticles-based materials. For example, the present invention relates to a method of intercalating a nanostructure or nanotube-containing material with a foreign species thereby causing the material to exhibit one or more of the following: reduction of the work function; reduction in the threshold electrical field for electron field emission; conversion of the semiconducting material to a metal; an increase in the electrical conductivity; an increase in the electron density of state at the Fermi level; and an increase the electron field emission site density.
In the description of the background of the present invention that follows reference is made to certain structures and methods, however, such references should not necessarily be construed as an admission that these structures and methods qualify as prior art under the applicable statutory provisions. Applicants reserve the right to demonstrate that any of the referenced subject matter does not constitute prior art with regard to the present invention.
The term xe2x80x9cnano-structuredxe2x80x9d or xe2x80x9cnanostructurexe2x80x9d material is used by those familiar with the art to designate materials including nanoparticles such as C60 fullerenes, fullerene-type concentric graphitic particles; nanowires/nanorods such as Si, Ge, SiOx, GeOx, or nanotubes composed of either single or multiple elements such as carbon, BxNy, CxByNz MoS2, and WS2. One of the common features of the xe2x80x9cnano-structuredxe2x80x9d or nanostructurexe2x80x9d materials is their basic building blocks. A single nanoparticle or a carbon nanotube has a dimension that is less than 500 nm at least in one direction. These types of materials have been shown to exhibit certain properties that have raised interest in a variety of applications.
U.S. Pat. No. 6,280,697 entitled xe2x80x9cNanotube-Based High Energy Material and Methodxe2x80x9d, the disclosure of which is incorporated herein by reference, in its entirety, discloses the fabrication of carbon-based nanotube materials and their use as a battery electrode material.
U.S. Pat. No. 6,630,772 entitled xe2x80x9cDevice Comprising Carbon Nanotube Field Emitter Structure and Process for Forming Devicexe2x80x9d the disclosure of which is incorporated herein by reference, in its entirety, discloses a carbon nanotube-based electron emitter structure.
Ser. No. 09/351,537 entitled xe2x80x9cDevice Comprising Thin Film Carbon Nanotube Electron Field Emitter Structurexe2x80x9d, the disclosure of which is incorporated herein by reference, in its entirety, discloses a carbon-nanotube field emitter structure having a high emitted current density.
U.S. Pat. No. 6,277,318 entitled xe2x80x9cMethod for Fabrication of Patterned Carbon Nanotube Films, the disclosure of which incorporated herein by reference, in its entirety, discloses a method of fabricating adherent, patterned carbon nanotube films onto a substrate.
U.S. Pat. No. 6,334,939 entitled xe2x80x9cNanostructure-Based High Energy Material and Method, the disclosure of which is incorporated herein by reference, in its entirety, discloses a nanostructure alloy with alkali metal as one of the components. Such materials are described as being useful in certain battery applications.
U.S. Pat. No. 6,553,096 entitled xe2x80x9cX-Ray Generating Mechanism Using Electron Field Emission Cathodexe2x80x9d, the disclosure of which is incorporated herein by reference, in its entirety, discloses an X-ray generating device incorporating nanostructure-containing material.
Ser. No. 09/817,164 entitled xe2x80x9cCoated Electrode With Enhanced Electron Emission And Ignition Characteristicsxe2x80x9d the disclosure of which is incorporated herein by reference, in its entirety, discloses an electrode including a first electrode material, an adhesion-promoting layer, and a carbon nanotube-containing material disposed on at least a portion of the adhesion promoting layer, as well as associated devices incorporating such an electrode.
As evidenced by the above, these materials have been shown to be excellent electron field emission materials. In this regard, such materials have been shown to possess low electron emission threshold applied field values, as well as high emitted electron current density capabilities, especially when compared with other conventional electron emission materials.
For example, it has been shown that the electronic work functions of carbon nanotube materials, which is one of the critical parameters that determines the electron emission threshold field, are in the range of 4.6-4.9 eV (electron Voltage). See, e.g.xe2x80x94xe2x80x9cWork Functions and Valence Band States of Pristine and Cs-intercalated Single-walled Carbon Nanotube Bundles,xe2x80x9d Suzuki et al, Appl. Phys. Lett., Vol. 76, No. 26, pp. 407-409, Jun. 26, 2000.
It has also been shown that the electronic work functions of carbon nanotube materials can be reduced substantially when they are intercalated with alkali metals, such as cesium. See, e.g.xe2x80x94Ibid., and xe2x80x9cEffects of Cs Deposition on the Field-emission Properties of Single-walled Carbon Nanotube Bundles,xe2x80x9d A. Wadhawan et al., Appl. Phys. Lett., 78 (No. 1), pp. 108-110, Jan. 1, 2001.
As illustrated in FIG. 1, the spectral intensity at the Fermi level of the pristine single walled carbon nanotubes is very small. On the other hand, a distinct Fermi edge is observed for the Cs-intercalated sample. From the spectral intensity at the Fermi level, we can conclude that the density of states at the Fermi level of the Cs-intercalated sample is roughly two orders larger than that of the pristine material. Further, as illustrated in FIG. 2, the results show that the work function of the single-walled carbon nanotube decreases with increasing Cs deposition time. (The spectra were measured at room temperature using a He lamp Hv=21.22 eV).
By reducing the electronic work functions of carbon nanotube materials, the magnitude of the applied electrical field necessary to induce electron emission can be significantly reduced. This relationship can be understood from the Fowler-Nordheim equation:
I=aV2exp(xe2x88x92bxcfx863/2/bV)
wherein I=emission current, V=applied voltage, xcfx86=electron work function, and xcex2=field enhancement factor, a and b=constants.
Thus, as evident from the above equation, a reduction in the work function value xcfx86, has an exponential effect on the emission current I. Experimental evidence has verified the above-noted relationship.
Nanotubes, such as carbon nanotubes synthesized by the current techniques such as laser ablation, chemical vapor deposition, and arc-discharge methods typically have enclosed structures, with hollow cores that are enclosed by the graphene shells on the side and ends. Carbon nanotubes, especially single-walled carbon nanotubes have very low defect and vacancy density on the side walls. The perfect graphene shells can not be penetrated by foreign species. The interior space of the nanotubes is usually inaccessible for filling and/or intercalation. Although defects are commonly observed on the sidewalls of the multi-walled carbon nanotubes, only the space between the concentric graphene shells is partially accessible.
Previous techniques for intercalating the carbon nanotube materials have included techniques such as vapor phase reaction between the raw carbon nanotube materials and the material to be intercalated (e.g.xe2x80x94alkali metal), and electrochemical methods. Examination of carbon nanotube materials intercalated in this manner has revealed that the alkali metal atoms intercalate into space between the single-walled nanotubes inside the nanotube bundles or the space between the concentric graphene shells in multi-walled carbon nanotubes.
However, such intercalated carbon nanotube materials possess certain disadvantages.
First, since alkali metals are extremely air-sensitive, the interaction with carbon nanotube materials must take place in a vacuum environment. This makes these materials difficult to process, and difficult to incorporate into practical devices.
Second, alkali metals have a relatively high vapor pressure and can be easily evaporated at relatively low temperatures. Thus, alkali metal which is deposited on carbon nanotube materials is very unstable during emission and can degrade easily in a short period of time, due at least in part to evaporation of the intercalated metals from the carbon nanotubes.
Third, carbon nanotubes are in the form of a closed cage-like structure, which typically possesses relatively few defects. This is especially true for single-walled carbon nanotubes. Thus, there is a tendency for the intercalated alkali metal atoms to be deposited within the bundles of nanotubes rather than inside the closed cage-like nanotubes themselves. This can be undesirable because the interior space of the nanotubes represents a much larger volume than the interstitial sites within carbon nanotube bundles. This limits the amount of metals which can be intercalated into the carbon nanotube materials.
Thus, there is a need in the art to address the above-mentioned disadvantages associated with methods for reducing the electronic work function of nanotube and nanoparticle materials, such as carbon nanotubes, particularly with regard to the technique for intercalating electron donors such as alkali metals, or even electron acceptors.
The present invention addresses the above-mentioned disadvantages associated with the state of the art, and others.
For example, the present invention provides a means for forming enclosed structures comprising alkali metals or other foreign species which are sealed within the nanotubes or nanoparticles themselves. These enclosed structures form xe2x80x9ccapsulesxe2x80x9d which are then stable in air and other environments, such as solvents. Since the alkali metal or foreign species is enclosed within these capsules, it is no longer as sensitive to the environment as would be the case if the material was exposed. Thus, the above-mentioned capsules can be further processed under ambient conditions. The intercalated alkali metals or other foreign species which are located within the capsules cannot be evaporated easily. Moreover, the intercalated alkali metal or foreign species within the capsules is less chemically reactive, i.e.xe2x80x94is relatively chemically inert. Because the interior space of the capsules are larger than the interstitial sites between adjacent structures, such as carbon nanotube bundles, more alkali metals or other foreign species may be stored. Due to charge transfer from the intercalated alkali metals to the surrounding strucutures, the resulting materials should have lower electronic work function values and higher densities of states at the Fermi electron level. In addition, because of the above-mentioned charge transfer phenomena, the capsules, which typically contain both semiconducting and metallic material, become all metallic in nature after the above-described intercalation performed according to the present invention. This charge transfer effect acts to further improve electron emissions properties.
According to one aspect, the present invention provides a method of manufacture comprising: (a) producing raw nanostructure or nanotube-containing material comprising closed structures; (b) purifying the raw material; (c) processing the purified material thereby forming openings in the closed structures; (d) introducing a foreign species comprising electron donors or electron acceptors into at least some of the openings; and (e) closing the openings, thereby forming capsules filled with the foreign species.
According to a further aspect, the present invention provides a method of reducing electronic work function, reducing threshold field emission values, converting semiconducting behavior to metallic behavior, increasing the electron density state at the fermi level, and increasing electron emission site density, of carbon nanotube-containing material, the method comprising: (a) forming openings in the carbon nanotube-containing material, (b) introducing a foreign species comprising an alkali metal into at least some of the openings, and (c) closing the openings, thereby forming carbon nanotube capsules filled with the foreign species.
According to another aspect, the present invention provides a method of manufacture comprising: (a) producing vertically oriented carbon nanotubes on a support surface;(b) applying an insulating layer;(c) opening tops of the nanotubes; (d) introducing a foreign species into the open tops and into interior spaces of the nanotubes;(e) closing the open tops of the nanotubes; and (f) activating the filled nanotubes.
The present invention also provides an electron field-emitting device comprising capsules formed by any of the methods described herein. The device can include, for example, an x-ray tube, a gas discharge device, a lighting device, a microwave amplifier, an ion gun, or an electron beam lithography device.
According to yet a further aspect, the present invention provides an article of manufacture comprising capsules formed by the any of the methods described herein. The article having an electron emission turn-on field to obtain an electron emission current density of 0.01 mA/cm2 of less than 2V/xcexcm.